We studied the changes in UV penetration associated with the dynamics of a shallow (mean depth = 0.6 m) coastal lagoon of South America that communicates periodically with the Atlantic Ocean. Two characteristic situations, i.e., freshwater dominance and salt-wedge intrusion were considered. Nine stations were sampled along the main axis of the lagoon, the main tributary, and the adjacent coast. The attenuation in the UV-B, UV-A and PAR wavebands were related to changes in the concentration of dissolved organic carbon (DOC), chlorophyll a (chl a), absorption (a d) of the chromophoric dissolved organic matter (CDOM), fluorescence of CDOM (F d), organic (OSS) and inorganic (ISS) suspended solids. The area most influenced by the marine intrusion showed the lowest DOC concentration (1.8 mg l-1) and the highest UV penetration. In this area, the depth corresponding to 10% of the irradiance below the surface (Z 10%) accounted for 66 and 100% of the water column for the UV-B and UV-A wavebands, respectively (diffuse attenuation coefficient, K d = 7.3 and 2.1 m-1). The other zones of the lagoon (DOC = 5.7 to 9.3 mg l-1) presented low UV-B penetration (K d = 29 to 64 m-1), and the Z 10% for UV-A accounted for 30 to 64% of the water column (K d = 7 to 14 m-1). Under both hydrological situations, the Z 10% for PAR reached the bottom in most parts of the lagoon (K d = 1 to 5 m-1). A significant reduction in DOC-specific absorption (a d*) during the marine intrusion indicated a change in the absorption characteristics of the CDOM. The variability in K d for UV-B radiation was mainly explained by the concentration of dissolved and particulate organic substances, while the K d values for UV-A and PAR were best predicted by the concentration of DOC